1. Field of the Invention
This invention relates to a cross polarization compensating system which removes cross polarization interference from two elliptically polarized waves of degraded cross polarization discrimination to provide correct, orthogonally polarized waves.
2. Description of the Prior Art
As a kind of radio communication, there is orthogonal dual-polarization communication which is intended to increase the channel capacity through utilization of orthogonal polarized waves of the same frequency, which are, for example, a vertically polarized wave and a horizontally polarized wave or a clockwise polarized wave and a counterclockwise polarized wave. When carrying this system into practice, it is necessary to minimize degradation of the cross polarization discrimination which occurs in a transmitting and receiving antenna system and on a propagation path. To this end, it is important to improve the cross polarization characteristic of the antenna system and to compensate for degraded cross polarization discrimination on the propagation path. The cross polarization which occurs on the propagation path is mostly caused by a raindrop. The harder rain becomes, the flatter the raindrop becomes, providing attenuation and phase differences between polarized waves which propagate in directions of the longer and the shorter axis of the raindrop. As a consequence, the orthogonality of the polarizations is lost and the polarized waves become elliptically polarized ones, resulting in the cross polarization discrimination being degraded. As a method for effectively compensating for such degraded cross polarization discrimination, there has heretofore been proposed a method in which cross polarization caused by the non-isotropy of the phase characteristic is compensated for by a first compensating part using two rotatable phase shifters, so that cross polarization caused by the non-isotropy of the attenuation characteristic is cancelled by a second compensating part. This prior art system is classified into the following two systems according to a difference in the operation of the first compensating part.
One of them is a system (I) which makes the major axes of two polarized waves perpendicular to each other; namely, two arbitrary elliptically polarized waves are converted by rotatable 90.degree. and 180.degree. phase shifters into elliptically polarized ones whose major axes are perpendicular to each other at the input of an Orthomode (Trademark) transducer (OMT). (See 1975 IEEE International AP-S Symposium Digest, P 209-212, "Adaptive Polarization Control for Satellite Frequency Reuse System"; and Japanese patent application Disclosure No. 115717/76).
The other system is one which converts one of the two polarized waves into a linearly polarized one. This system is characterized by conversion of two incoming elliptically polarized waves into a linearly polarized one. (See 1977 IEEE International AP-S Symposium, PP 173-176, "Broadband Adaptively Controlled Polarization Network"). As for the phase shifters, there have been proposed a system of combining 90.degree. and 180.degree. phase shifters and a system (II) of using two 90.degree. phase shifters. (See Japanese patent applications Disclosure No. 97719/78 and U.S. Pat. application Ser. No. 007,463 filed Jan. 29, 1979).
In the above conventional system, the set angles of the phase shifters are not uniquely determined with respect to orthogonal, circularly polarized waves.